argh.rb

require 'active_record'
require 'minitest/autorun'

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

class Message < ActiveRecord::Base
  connection.create_table(table_name)
  connection.create_table :message_threads_messages, id: false do |t|
    t.references :message
    t.references :message_thread
  end

  has_and_belongs_to_many :message_threads, before_add: :callme
  has_many :topics
  has_many :people, through: :topics, before_add: :callme

  def callme(model)
    p :NEW => [model.class, model.new_record?]
  end
end

class Topic < ActiveRecord::Base
  connection.create_table(table_name) { |t|
    t.references :message
    t.references :person
  }
  belongs_to :person
  belongs_to :message
end

class Person < ActiveRecord::Base
  connection.create_table(table_name)
  has_many :topics
end

class MessageThread < ActiveRecord::Base
  connection.create_table(table_name) { |t| t.references :latest_message }
  has_and_belongs_to_many :messages
  belongs_to :latest_message, class_name: 'Message'
end

class BugTest < MiniTest::Unit::TestCase
  def test_before_add
    m = Message.create!
    puts "HABTM"
    m.message_threads << MessageThread.new
    m.topics << Topic.new
    puts "HM:T"
    m.people << Person.new
  end
end

output.markdown

4-0-stable

HABTM
{:NEW=>[MessageThread(id: integer, latest_message_id: integer), true]}
HM:T
{:NEW=>[Person(id: integer), false]}
.

master

HABTM
{:NEW=>[MessageThread(id: integer, latest_message_id: integer), false]}
HM:T
{:NEW=>[Person(id: integer), false]}
.